AWS IOT - Credential should be scoped to correct service

Sign your request with iotdata instead if iot
example:

AWSSignerForAuthorizationHeader signer = new AWSSignerForAuthorizationHeader(
    endpointUrl, "GET", "iotdata", regionName);

In your 4th step, don't fill anything for Service Name. Postman will default the value with execute-api.

Hope this works!